If you have ever pulled a rock out of your tank, scrubbed off every strand of green, and watched it come back thicker two weeks later, you have probably been fighting bryopsis and treating it like hair algae. They are not the same organism, and the thing that beats one barely annoys the other.
Telling them apart
Get your face close to the glass, or better, take a macro photo and zoom in. That is where the difference lives.
- Green hair algae (GHA) grows as fine, unbranched filaments. Individual strands look like wet hair — smooth, tapering, no structure. It grips substrate and rock loosely and comes away in clumps when you pull it.
- Bryopsis is a green alga with a distinct feather or fern pattern. Each frond has a central stalk with side branches coming off it in a flat plane. Reefers often describe it as looking “pinnate” or like tiny pine branches. It anchors hard, and it snaps rather than slides when you pull.
There is a second, uglier tell: bryopsis is usually the one your herbivores refuse to touch. If your tangs, urchins and hermits mow down everything except one patch, look closely at that patch. Bryopsis produces compounds that make it unpalatable to most grazers, which is exactly why the “just add more herbivores” advice fails on it.
Why manual removal fails on bryopsis
Pulling GHA out works because you are removing most of the biomass and the rest gets grazed or starved. Bryopsis regenerates aggressively from fragments and holdfast material left in the rock. Worse, ripping it releases fragments into the water column that settle and start new patches. Plenty of reefers have turned one corner of bryopsis into a tank-wide problem in a single enthusiastic cleaning session.
If you are going to manually remove bryopsis, do it with a siphon running so fragments leave the tank instead of relocating in it, and accept that removal is cosmetic, not curative.
What actually works on each
For green hair algae, GHA is a nutrient and grazing problem, and the fix is boring and reliable: get nitrate and phosphate to sane, stable levels rather than zero, improve flow in dead spots, export with a decent skimmer or refugium, and put the right grazers in. GHA is genuinely beatable by husbandry alone. Most tanks that stay clean stay clean because of export, not because of a product.
For bryopsis, husbandry alone usually is not enough, which is the frustrating part. The two approaches with real track records are elevated magnesium held well above normal reef levels for a sustained period, and fluconazole dosing. Both are whole-tank interventions with real caveats — fluconazole is an antifungal that will affect more than your target algae, high magnesium is stressful to some inverts and easy to get wrong, and either one requires you to stop running carbon and be careful about water changes for the duration. This is not a “pour it in and see” situation. Read a real protocol before you start, and know what your baseline parameters are so you can tell what changed.
The prevention that costs nothing
Almost every bryopsis outbreak arrives on something: a frag plug, a piece of live rock, a snail shell. It does not spontaneously appear in a clean system. Quarantining and dipping incoming corals is the single cheapest control you have, because it is far easier to inspect and treat one frag in a cup than an entire display.
Inspect the plug and the base, not just the coral. Look at the underside. If you see feathery green structure, cut it off, or better, do not put that plug in your tank at all — mount the coral on a new plug and discard the old one.
Get the ID right the first time
Misidentification is what costs people months. Bryopsis, derbesia, cladophora, dinos and cyano all get called “algae” and all respond to completely different treatments, and the wrong treatment loses you weeks and sometimes livestock.
Our Algae & Flora ID Guide is a 25-page side-by-side reference — photos of the common nuisance algae and cyanobacteria at the magnification you would actually see them, what each one indicates about your system, and what to do about each. It is an instant PDF download, so you can have it open on your phone at the glass in about thirty seconds.
If pests are your real problem too, the Pest & Disease Bundle pairs it with the coral pest and fish disease guides, or you can get everything in the Complete Reef Care Library.